1. Field of the Invention
This invention relates to limb tip cam pulleys for a compound archery bow and more particularly to cam pulleys mounted on the tips of the bow limbs engaged by the bowstring and take-up strings which cam pulleys are designed for high energy operation of the bow.
2. Archery Bow Action
Drawing of an archery bowstring by an archer effects bending of resilient bow limbs extending oppositely from a grip in opposition to their bias to straighten. Such bending of the bow limbs produces a storage of potential energy in the stressed bent bow limbs which upon release of the bowstring unflexes the bow limbs to straighten the bowstring which is in engagement with the nock of the arrow. Such straightening of the bowstring from bent position drives the arrow. The greater the force required to bend the bow limbs to a given degree and the greater the degree of bend, the more potential energy will be stored in the bow limbs. Consequently, the greater will be the energy available for driving the arrow when the bowstring is released and the greater will be the acceleration and resultant speed of the arrow so that it will travel a greater distance and/or will strike a target with greater force.
The amount of potential energy that is stored by the bent bow limbs is related directly to the amount of force that is required to draw the bowstring back and the distance that the bowstring is pulled. The greatest potential energy would be produced if the maximum draw force which a particular archer is capable of exerting were maintained constant at all stages of the draw, but various considerations make such a constant draw force throughout the entire extent of the bowstring nocking point draw displacement impractical and undesirable.
In a longbow in which the opposite ends of a bowstring are fixedly attached to the ends of two bow limbs that are substantially symmetrical about the bow grip, the draw force increases progressively as the bowstring is drawn, so that at full or maximum draw the draw force also is maximum. Since the arrow must be sighted when the bow is fully drawn, the requirement for maximum draw force at the maximum draw distance severely limits the power of the bow that longbow archers could shoot comfortably and accurately.
Various types of compound bow have been devised, a particular objective of all of which is to provide a bow construction enabling the bow to be held at full draw of the bowstring by a force less than the force required to be exerted on the bowstring at some intermediate point in the draw to reach full draw position. Different types of bows have different draw force requirements for drawing the bowstring from the position of rest to the fully drawn position.
3. Prior Art
An early compound bow enabling the bowstring to be held in fully drawn position by exerting on it a force less than the force required at an intermediate position of the draw is disclosed in Allen U.S. Pat. No. 3,486,495. The bow of this patent used pulleys pivotally mounted on the tips of flexible bow limbs which pulleys were engaged by the bowstring and by take-up strings extending from a pulley on one limb tip to an anchor on the other limb tip. In one instance, the pulleys were generally of oval profile and in a modification the pulleys were circular. In both instances the pulleys were journaled at a location offset from the center of the geometric shape. In both instances the pulleys had two pulley components disposed in registration, one for the bowstring and the other for a take-up string, and both pulley components were of substantially the same configuration.
Curves plotting draw force as ordinates and bowstring nocking point draw displacement as abscissae portrayed a generally hyperbolic curve in which the force required to draw the bowstring during initial displacement of the nocking point increases rapidly, as the intermediate position is approached the required force increases less rapidly until a maximum is reached at approximately mid draw, and the draw force then decreases until full draw is reached so that the maximum force applied by the archer to the bowstring will not be required to hold the bowstring in the fully drawn position. The Allen bow is stated to have an increase in energy over the longbow without increasing the length of the draw or the holding force required in the fully drawn position.
An archery bow generally similar to the bow of Allen U.S. Pat. No. 3,486,495 is disclosed in U.S. Pat. No. 4,060,066 of Kudlacek. This patent states that previous compound bows had utilized paired cam elements with their cam elements concentrically joined together, whereas in the Kudlacek bow each cam member comprised dual cam elements secured together eccentrically. Each of the cam elements is in the form of a circular pulley provided with a single peripheral guide groove. Use of such eccentrically mounted cam elements operated to provide a draw force or weight which varied with the extent of the draw. The patent compares the operation of its bow with those of prior compound bows having concentrically mounted circular cam elements and points out that by utilizing circular cams of different size in which the pivot of the cam of one size is offset from the pivot of the cam of the other size, the maximum draw force point for the Kudlacek patent bow occurs earlier in the draw although the maximum draw force is approximately the same. Such feature of having the maximum draw force occur at approximately 2 inches (5 cm) less draw displacement is stated to be especially advantageous for persons having short arms. Also, the draw force to draw distance curve is flatter in the full draw region making it easier for the archer to arrive at and maintain full draw during sighting and shooting.
The Kudlacek patent points out that the total area under the draw force curve is greater for the Kudlacek bow than for the prior art bow, presumably the bow of Allen U.S. Pat. No. 3,486,495, representing greater total potential energy and consequently providing greater arrow speed with increased accuracy and distance.
The later Barna, U.S. Pat. No. 4,202,316 discloses an archery bow of the same general type as shown in Allen, U.S. Pat. No. 3,486,495 and in Kudlacek, U.S. Pat. No. 4,060,066, but, in this instance, the pulley means mounted on each bow limb tip includes only a single pulley of circular profile which is pivoted eccentrically. Each pulley includes sockets on its outer circumference engageable by beads on the bowstring. Such beads prevent the bowstring from slipping on the pulley. A particular object of the bow of this patent is to mount the bowstring on the pulleys in a manner which removes the bowstring from contact with the fletching of a released arrow without subjecting the limbs of the bow to a torque. Also, the draw force and draw length of the bow are adjustable by altering the length of the flexible string portion to move the limb free end portions toward or away from each other.
A still later patent disclosing the same general type of compound archery bow is Jennings, U.S. Pat. No. 4,241,715. The pulley means mounted on the tips of the bow limbs in this instance are eccentric circular pulleys. Each pulley means includes a larger circular pulley and a smaller circular pulley that are fixed in concentric relationship and which composite pulley is pivotally mounted for turning about an axis offset from the center of the pulleys. While the two pulley components of each composite pulley are shown as being of different size, Jennings states that the diameters of the two components may be the same.
The string passes diagonally through the Jennings composite pulley from one circular pulley section to the other. The position of the string passage enables the draw length and draw force of the bow to be adjusted. This patent is not concerned with the relationship between the draw force and the degree of bowstring nocking point draw displacement at different phases of the draw.
None of the Allen, Kudlacek, Barna and Jennings patents is particularly concerned about providing a bow that will store the greatest practical amount of potential energy while being drawn with a given maximum drawing force. The Allen U.S. Pat. No. 3,486,495 states that the bow of that patent requires the archer to apply added force at the commencement of the draw to effect an increased energy buildup so that at full draw greater energy will be imparted to the limbs although a lesser force is required to hold the bowstring.
The Alexander U.S. Pat. No. 3,851,638 discloses a compound bow different from the general type shown in Allen, U.S. Pat. No. 3,486,495, Kudlacek, U.S. Pat. No. 4,060,066, Barna, U.S. Pat. No. 4,202,316, and Jennings, U.S. Pat. No. 4,241,715. The bow of this patent also, however, is concerned with providing a leverage system which will enable the bow to be held more easily in fully drawn position while being aimed, which was an objective of the Allen U.S. Pat. No. 3,486,495. The Alexander patent states at column 4, lines 58 to 64, "as the bowstring is drawn back, the combined actions of the bowstring and the power cams produce an increase in leverage, reducing the amount of force required to draw the bow. The draw force diminishes steadily from a maximum at the rest point of bowstring 44 to a minimum at the fully drawn position." The Alexander bow thus allows an archer to use a more powerful bow than a longbow because the maximum pull is required at the start of the draw where the archer's hands are close together and he is able to exert maximum leverage instead of at the end of the draw when one arm is highly flexed. Such a bow, however, does not produce maximum energy because of the steady decrease in force required to draw the bow as the degree of draw progresses.
A different type of compound archery bow is disclosed in Trotter, U.S. Pat. No. 3,923,035. Trotter states that an object of his invention is to provide a compound bow in which the bowstring tension initially sharply increases at the beginning of the draw to a maximum design tension, and this maximum tension is maintained until full draw of the bow is approached, at which point the maximum design tension drops to an optimum design holding tension at full draw. It is pointed out that various compound bow designs had been proposed in order to achieve the combination of a powerful arrow propulsion system and optimum bowstring tension at full draw. The Trotter bow is, however, much more complicated than the compound bows of the Allen, Kudlacek, Barna and Jennings patents discussed above.